1. Field of the Invention
The present invention relates generally to inkjet recording apparatuses, and further to a driving apparatus for an inkjet recording apparatus.
2. Description of the Related Art
Among conventional printer heads for an inkjet printer, there are known a head which ejects ink by pressurizing the ink channel using a piezoelectric element, and a head which ejects ink by evaporating ink in the ink channel using a heater element. The former printer head causes distortion in the piezoelectric element by application of pulse voltage. The distortion pressurizes the ink in the ink channel, and causes a drop of the ink ejected from a nozzle in communication with the channel. By repetition of ink drop ejection, an ink image is formed on a recording sheet.
The latter printer head applies pulse voltage to the heater element provided in the ink channel to heat the same. The heat generated from the heater element partly evaporates the ink in the ink channel. Taking advantage of the expansion in volume caused by the evaporation, a drop of the ink is ejected from a nozzle. By repetition of ink drop ejection, an ink image is formed on a recording sheet.
In both printer heads, the pressure within the ink channel abruptly changes by the driving of the pulse voltage. The abrupt pressure change causes a wave in the ink channel after a drop of the ink is ejected. If the next pulse voltage is applied with the wave still remaining, the diameter (size) of each drop of the ink ejected from the nozzle is changed because of the waves, which causes variations in printed characters and results in significant degradation of the picture quality.
Although the waves are attenuated with time, if the next pulse voltage is applied after the attenuation of the waves, the printing speed is significantly lowered, contrary to the demand of high speed operation in recent years.
There is a known printer using a printer head of the former type, in other words using a piezoelectric element, which applies a sub pulse voltage immediately before applying a main pulse voltage to the piezoelectric element. In the printer, the ink in the ink channel is allowed to positively vibrate by applying the sub pulse voltage. The force created by the vibration together with the pressure created within the ink channel by the application of the main pulse voltage permit drops of the ink to be efficiently ejected from the nozzle.
Another printer of the former type, in other words a printer using a piezoelectric element, is known which applies sub pulse voltage immediately after applying a main pulse voltage to the piezoelectric element. In the printer, waves caused after a drop of ink is ejected, are prevented by applying the sub pulse voltage. More specifically, after the elapse of time almost equal to the pulse width (time) of the main pulse voltage since the main pulse voltage is stopped, the sub pulse voltage is applied.
Furthermore, there is known a printer of the latter type, in other words a printer using a heater element, which applies sub pulse voltage immediately before applying main pulse voltage. In the printer, the heater element is elevated in temperature by applying the sub pulse, so that the diameter of each drop of ink to be ejected from the nozzle is stabilized by the main pulse voltage applied after the sub pulse.
Any of the conventional ink jet printers employing the application of the sub pulse voltage does not address the adverse effect of a wave caused in the ink channel on the formation of the next drop of the ink.